Landline telephony uses supervision to detect changes in the switch-hook state caused by the telephone user. Mobile telephone supervision also performs this process but must also ensure that adequate RF signal strength and interference protection is maintained. This is accomplished by the supervisory audio tone (SAT), a continuous out-of-band modulated signal.
Three SAT signals are used in the United States cellular system, AMPS. These SAT signals are at 5970 Hz, 6000 Hz, and 6030 Hz. Only one of these frequencies is employed at a given cell site.
The SAT operates by the mobile unit receiving the SAT from the base station and transponding it back to close the loop. The base station looks for the return of the specific SAT it sent out. If another SAT is returned, the cell interprets this as the call between the mobile and the cell being corrupted by interference.
When a mobile receives a signal from the base station, it detects whether SAT is present. Based on this frequency, the mobile generates its own SAT and transmits it back to the base station. This method requires the mobile to perform a large number of million instructions per second (MIPS) in order to detect the received SAT and generate the transmitted SAT.
Consumers are demanding smaller cellular telephones for greater portability. To reduce the size of the telephones, the number of parts in the telephone must be reduced. This can be accomplished by performing many of the telephone's functions in a digital signal processor (DSP). This, in effect, replaces a number of integrated circuits with a single DSP that performs the same function as the replaced ICs.
Replacing the present SAT detection and generation circuits with a DSP would require a very MIPS intensive process. This would take up time the DSP could be used for other tasks. Additionally, the received SAT must be adjusted for gain differences caused by the mobile's hardware before it is transmitted back to the base station. This would take additional time away from the DSP. There is a resulting need for a simple process to detect SAT and adjust its gain.